288 SYDNEY E. JOHNSON 
that there are from fifteen to twenty clusters of hair cells for 
every vertebral segment. Evidence based on the arrangement 
of the lateral ramuli and the surface tubules is unsatisfactory 
partly for the same reason and partly for other reasons. As 
shown above, the lateral ramuli and the surface tubules are 
considerably nore numerous than the vertebral segments and a 
constant ratio between the number of vertebrae and ramuli of the 
lateral nerve is wanting. Furthermore, these ramuli are merely 
the branches of distribution of a cranial nerve which differs from 
other cranial nerves only because of the fact that it supplies this 
remarkable type of sense organ and extends from the head to the 
caudal fin. In this connection it must be remembered that the 
fibers of the ramuli diverge at the base of the sensory epithelium . 
to form a continuous fiber zone from which the ultimate dis- 
tribution takes place. 
Further difficulty is met in attempting to relate the numerous 
organs of the head canals and of the cross-commissures to a cor- 
responding number of ancestral segments. 
In view of these considerations it seems improbable to me that 
the organs of the sensory canals have a phylogenetic history which 
would relate them either to the segmental sense organs of certain 
invertebrates, as claimed by Beard, Gaskell, and others, or to 
the posterior (body) segments of primitive vertebrates. To 
assume that the lateral sense organs have had such a past history 
involves the necessity of explaining why the innervation of the 
body organs should change from a segmental spinal nerve supply 
to a cranial nerve supply, and also, why the organs do not arise 
in situ on each segment of the body rather than from cephalic 
ectoderm which invades the posterior segments and carries with 
it its own nerve supply, probably from a corresponding primitive 
cephalic segment. It appears to me more likely that if the lateral 
sensory apparatus is segmental it is so only in relation to a limited 
number of cephalic segments. The several lines of organs, then, 
would represent simply an invasion or extension of a primitive 
cephalic sensory apparatus into other segments of the body. 
Clearly the evidence at hand is not sufficient to warrant 
dogmatic statements or conclusions. The need is emphasized 
